Google announced two months ago that it was going to start charging the heaviest users of its Maps API, which countless sites use to geo-locate their data. Then its sales team fanned out to contact those websites, which Google publicly estimated would represent only 0.35 percent of the users of its Maps API.

In what seems to have been a surprise to everyone, the prices that Google asked of its heaviest Maps users apparently dwarf the revenue of at least some of those sites, which is leading to a very public move away from Google and to OpenStreetMap.

Ed Freyfogle, cofounder of UK property search engine Nestoria, writes in a post on his company’s switch from Google to OpenStreetMap that despite its being run almost entirely on volunteer labor, and as a nonprofit, the free (as in software) alternative to Google Maps is every bit the equal of what the search giant has managed to assemble.

OpenStreetMap’s great strength is that anyone can contibute. Since the project started over 500,000 people around the world have signed up to do just that, often going into insane levels of detail. Fixes can be added and reflected in the maps very quickly. It is a fundamentally different model than the traditional “only and expert from the government can come make the map” model.

It probably doesn’t hurt that the price Google quoted for Nestoria to continue using its Maps API on just one of the eight websites it runs “would have bankrupted our company.”

That doesn’t mean that OpenStreetMap is necessarily free. The foundation behind OpenStreetMap can’t afford to serve maps to everyone who wants them, but the data it hosts can be copied and re-displayed by anyone who cares to.

This has led Google Maps competitor MapQuest to try and leverage OpenStreetMaps against Google by offering to serve free map tiles to whomever needs them, through its “mapquest open” service. Microsoft is getting in on the action too—last year it added an OpenStreetMaps layer to Bing, its search engine.

Freyfogle advises companies or developers thinking about making the switch to OpenStreetMaps to use Mapstraction, a “javascript mapping abstraction layer” that allows anyone who codes with it to easily switch between ten different mapping services. Building Nestoria in Mapstraction from the very beginning is one of the reasons that the site can be switched off of Google Maps so easily, he notes.

It’s doubtful that Google realized when it decided to start charging companies to use its Maps API that OpenStreetMaps was mature enough for developers to immediately jump ship. According to its fans, OpenStreetMaps isn’t just a viable alternative to Google Maps; it’s better. Some areas, particularly in Europe, where contributing to OpenStreetMaps is something of a craze, its maps are updated more frequently than Google’s maps, and contain vastly more detail.

Here’s an impressive 2008 animation of edits to OpenStreetMap produced by itoworld.com.